1. Insert microSD into MacPro Retina mmc-slot
2. use Disk Utility on Mac to Initialize/Erase the microSD. Format as MS-DOS, name is irrelevant
3. use Disk Utility to unmount just the UNTITLED partition
4. go to Terminal in Mac
5. you can optionally unmount the disk by doing:-

diskutil unmount /dev/disk1s1

6. issue:-

sudo dd bs=1m if=<img file name here> of=/dev/disk1

7. use ctrl-T to check status while writing out to microSD
8. use FileManager to Eject the card "BOOT" when done